The invention relates to a vertical axis wind turbine for extracting energy from wind. The invention also relates to a method for operating such a wind turbine.

A wind turbine is a machine that converts the kinetic energy from the wind into mechanical energy. Such mechanical energy when captured by a wind turbine may be employed to drive machinery using the abundantly available wind. The most familiar wind turbines to the general public are wind turbines which are essentially horizontal-axis wind turbines which have the main rotor shaft engaged to blades which are situated at the distal end of a large and tall tower (mast). The blades are engaged to the rotor at a substantially perpendicular angle and in order to spin the rotor the blades must be pointed in a direction into the wind. Smaller windmill type turbines are constantly redirected into the wind stream by a simple wind vane. Larger windmills or horizontal axis turbines being heavier generally employ a wind sensor coupled to a motor to constantly reposition the blades to intersect the wind stream driving them.

Horizontal axis style turbines have, at least in certain situations, a number of disadvantages. First they have difficulty operating in the light winds near the ground and must be elevated and employ tall towers able to support the force of the wind against long blades. Further because of their height and weight, horizontal axis turbines are difficult to install and maintain. Additionally, the blades have to be repositioned periodically to point into the wind which is rather laborious and easily leads to a reduced energy yield.

Vertical axis wind turbines (VAWTs) on the other hand, have the main rotor shaft running vertically. This arrangement has a key advantage over the horizontal axis turbine in that the generator can be placed at the bottom, near the ground so the tower doesn't need to support it. Further, vertical axis turbines do not require very large blades and tall towers to support them and the blades do not need to be constantly repositioned to point into the wind.

Power for vertical-axis wind turbines is generally provided by wind acting against the plurality of wing-shaped blades and the lift created by the wind passing over the surfaces thereof. One surface being longer than the other will create a lifting force as the wind traverses and accelerates to reach the rear of the blade at the same time as the wind traveling over the shorter surface. The lift created is perpendicular to the direction of the wind and therefore it is advantageous to reposition each blade to maximize lift during traverse through the airstream and minimize drag when rotated out of a perpendicular encounter with the same airstream.

Since vertical axis wind turbines operate closer to the ground and are simpler to assemble and install, vertical axis wind turbines are much easier to employ on a small scale to power homes and small businesses. Additionally, they do not present the large eyesore that conventional horizontal axis wind turbines exhibit. Finally, vertical axis wind turbines are much safer for wildlife such as birds which frequently fall prey to the large rotating blades of horizontal axis wind turbines since the low- positioned smaller blades are not encountered by unsuspecting wildlife in flight.

As such there is an unmet need for an improved design for a vertical axis wind turbine to allow wider employment of such devices to produce energy.

In this respect, before explaining at least one embodiment of the invention in detail it is to be understood that the invention is not limited in its application to the details of construction and to the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for designing other vertical-axis wind turbines and methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent construction insofar as they do not depart from the spirit and scope of the present invention.

An object of this invention is the provision of an improved vertical-axis wind turbine as well as a method for operating such turbine. These together with other objects and advantages which will become subsequently apparent reside in the details of the construction and operation as more fully hereinafter described and claimed, with reference to the accompanying drawings. The present invention proposes a wind turbine, comprising a substantially vertically oriented rotation shaft, at least one wing or blade that is attached to said rotation shaft for rotating the shaft when there is a wind force exerted to the wing or blade, at least one permanent magnet electromagnetic generator, attached or coupled to the shaft, at least one induction electromagnetic generator attached or coupled to the shaft, an electric power converter, for converting the power generated by the one permanent magnet electromagnetic generator and/or the induction

electromagnetic generator to a suitable voltage and/or waveform, a controller, configured for enabling the stator current of the at least one induction

electromagnetic generator when the rotation shaft is above a predetermined rotational speed; and disabling stator current field of the at least one induction electromagnetic generator when the rotation shaft is below a predetermined rotational speed.

A permanent magnet electromagnetic generator is to be understood as a generator having a rotor or stator comprising permanent magnets for causing a magnetic field, meaning that no external excitation for a field is required. Such generator directly starts to generate electric power once it is rotated.

An induction electromagnetic generator is to be understood as a generator having a rotor or stator comprising coils for causing a magnetic field, meaning that an external excitation for a field is required. Such generator only generates electric power once it is powered.

A disadvantage of permanent magnet generators is that they require a relatively high torque to overcome the default mutual orientation of the stator and the rotor, as a result of the magnetic field induced by the permanent magnets. This torque becomes higher when a larger output power of the generator is required, and therefore the permanent magnet generator is considered less suitable for high power than an induction electromagnetic generator. Another disadvantage is that the generated power as a result of the rotational speed has an asymptote, which makes it less efficient for high speed power generation. An induction electromagnetic generator has the advantage that a very low torque is required to rotate it when the field coils are not powered.

The generator according to the invention now combines both types of generators in an efficient way. The permanent magnet generator is used for low speeds, where it is beneficial since it does not require an external power source to generate power. At high speeds, where a permanent magnet generator has an asymptotic power yield that is limited, the induction electromagnetic generator takes over, since this type of generator becomes more effective at higher speeds. According to the invention, the power for the field of the induction electromagnetic generator may be provided by the permanent magnet generator.

Since a general disadvantage of permanent magnet generators is their relatively high friction torque for startup, according to the present invention, a relatively small permanent magnet generator is preferred. In particular, in a preferred embodiment of the present invention, a permanent magnet electromagnetic generator has a static torque smaller than 50 Nm and preferably smaller than 30 Nm. Generators with a maximum generated power of about 1 kW usually fulfil this requirement. In a further embodiment, the controller deducts the predetermined speed from the voltage generated by the permanent magnet electromagnetic generator. With a known voltage-speed curve, the voltage is a reference for the speed of the central shaft. An advantage is that this voltage, being an electric quantity, is directly applicable as an input signal for the converter. The presence of a generated voltage is also an indication that the generator is actually turning.

The controller may further be configured to electrically disconnect the permanent magnet electromagnetic generator from the power converter when the voltage generated by the permanent magnet electromagnetic generator exceeds a predetermined value. For this voltage, the voltage is chosen above which the permanent magnet generator is no longer efficient.

The controller may connect the permanent magnet electromagnetic generator to a battery charger for charging a battery that can be used for an auto-start of the permanent magnet generator, or to a resistance in the case that it is desired to let the permanent magnet generator function as a brake. In a more advanced embodiment, the wind turbine according to the invention comprises multiple induction electromagnetic generators, each with a different power level, wherein the controller is configured for sequentially enabling the stator fields of the induction electromagnetic generators. This way, the power level of the generator can be gradually adapted to the rotational speed of the generator, and thus to the wind speed. This enables to use each generator or each combination of generators at their maximum efficiency.

Preferably the induction electromagnetic generators have different power ratings, and the controller is configured for enabling higher power induction electromagnetic generators with increasing rotational speed of the shaft. Additionally, the controller may be configured for disabling lower power induction electromagnetic generators with increasing rotational speed of the shaft. The power converter may be configured for converting the generated power to a grid voltage, waveform and/or frequency, in order to be able to deliver the power to the grid. Alternatively, a battery may be provided, to which the generated energy is delivered, for later use. The controller may also be configured for disabling the stator current of at least one induction electromagnetic generator, when the rotational speed of the rotation shaft is within a predetermined range of an Eigen frequency of the turbine. By disabling the stator current, the torque required for rotating the generator decreases, and the central shaft encounters a lower load. As a result, its rotational speed will increase, and it will at a certain point get outside the predetermined range. From then on, the induction electromagnetic generator may be switched on again. This may be done gradually, to prevent lowering the rotational speed into the range again.

The controller may also be configured for transferring energy to a resistor when the wind speed exceeds a predetermined value, for example 25 m/s. This resistor dissipates energy and thus works as a break. Alternatively, the turbine may comprise a battery, coupled to the permanent magnet electromagnetic generator, for storing the generated energy therein. The battery may be arranged within a housing, such as a mast housing, of the permanent magnet electromagnetic generator. In a further embodiment, the turbine is provided with a fail-safe protection, comprising a timer, that electronically resets after a disruption. If the in mast battery, the grid and/or an additional energy storage cannot supply power, a short circuit used as a brake is never disengaged after the specific time period, because the timer will not work without power. The turbine can become operational again only when the short circuit brake is manually disengaged.

The invention will now be elucidated into more detail with reference to figure 1 , which shows a schematic view of an embodiment of the present invention. The figure shows a wind turbine 1 , comprising a substantially vertically oriented rotation shaft 2, a number of blades 3 that are attached to said rotation shaft 2 for rotating the shaft 2 when there is a wind force exerted to the wing or blade. The turbine has a permanent magnet electromagnetic generator 4 coupled to the shaft 2 and an induction electromagnetic generator 5 attached or coupled to the shaft 2.

The wind turbine further comprises an electric power converter 6, in this case for converting the power generated by the induction electromagnetic generator 5 to a suitable voltage and waveform, in this case for a grid 8. In other embodiments, the electric power converter 6 may also convert the power from the permanent magnet electromagnetic generator 4 to a value suitable for the grid 8. Here however, a second power converter 9 is present, for converting the power from the permanent magnet electromagnetic generator 4 to a voltage suitable for a battery charger 10, to which a battery 1 1 is coupled. The wind turbine further comprises a controller 7, configured for enabling the stator current of the induction electromagnetic generator 5 when the rotation shaft 2 is above a predetermined rotational speed; and disabling stator current field of the induction electromagnetic generator 5 when the rotation shaft 2 is below a predetermined rotational speed. The controller 7 deducts the predetermined speed from the voltage generated by the permanent magnet electromagnetic generator. Thereto, a voltage sensing array 12 is coupled between the permanent magnet electromagnetic generator 4 and the controller 7.

Additionally or alternatively to the grid 8, the power converter 6 may be coupled to an external energy storage 13, for storing (a surplus of) energy therein. A further addition may be a second induction electromagnetic generator 5' which may be switched on in dependency of the required power, the wind speed and/or the rotational speed of the shaft 2. This embodiment was presented as an example only and does in no way or sense limit the scope of protection of the present application, as defined in the following claims.